GB1212696A - Improvements in or relating to nuclear reactors - Google Patents

Abstract

1,212,696. Reactors. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. 11 Dec., 1968 [21 Dec., 1967], No. 58245/67. Heading G6C. A reactor core comprises a number of prismoidal columns of moderator material bearing nuclear fuel, coolant passages for passing coolant in heat extracting relation with the fuel, the arrangement being such that the fuel is replaceable by replacing at least some of the columns or parts thereof, and a series of longitudinal members in channels extending parallel to and between at least some of the adjacent columns, at least alternate ones of the members being removable and once removed allow replacement only of those columns adjacent to them, whilst the other columns are retained in interlocking relationship by virtue of the remaining longitudinal members and the prismoidal shape of the columns. In one embodiment the columns are formed of graphite blocks each having an irregular, substantially pentagonal, cross-section (as shown in Fig. 2). Adjacent sides 12, 16 and 13, 14 are at right angles to one another, whereas the angles included by the other pairs of adjacent sides are each 120 degrees. At the corners between the two opposite pairs of adjacent sides 12, 16 and 13, 14, the edge of the block is cut away by arcuate grooves 17a, 17b, the surfaces of which form one quadrant of a circular section hole C,S when similar corners of four such blocks are placed in juxtaposition (as shown in Fig. 3). The shape of the blocks is such that each group of four blocks which co-operate to define any given circular hole lie on a square lattice. The holes C,S contain a longitudinal core member that is either a control rod or a spreader rod. The latter is a rod or tube of low neutron absorption cross-section, preferably with a lead-in taper at one end and perhaps a slight taper along its shank. Clearance between faces 12, 16 of adjacent blocks is provided so as to facilitate withdrawal and replacement of blocks during refuelling, even if irradiation-induced effects have caused distortion of the blocks, and relative movements due to this clearance are prevented during normal operation by the spreader rod. This is removed prior to refuelling. Each block is drilled with outer and inner annular rows of holes 21, 22 which house fuel compacts and allow for the passage of a coolant gas. A central hole 23 has three radial slots 24 which allow the expandable fingers of a fuel handling tool to engage the underside of the block once it has entered the hole 23. The fuel compacts each comprise a graphite tube with an annular fuel filling within its wall thickness. The base of the tube is screw-threaded to engage complementary screw-threads on the inner ends of three spaced fingers which project inwardly from around the lower end of each of the holes 21, 22. A stopper at the upper end of the tube has spaced projections which serve to centralize the top of the tube. The blocks spigot into one another with a spacing between the adjacent horizontal faces of the blocks to allow for distortion. The tube bores form coolant channels, the adjacent regions of superimposed channels being arranged with suitable fairing so that there is a reduction in coolant velocity at these junctions. Consequently there is a reduction in velocity head of the coolant gas and associated losses will likewise be reduced.